1. Technical Field
The present invention relates to an etching composition for texturing a crystalline silicon wafer and a method of etching a crystalline silicon wafer using the etching composition. More particularly, the present invention relates to an etching composition for texturing a crystalline silicon wafer, which can uniformly texture the surface of a crystalline silicon wafer in a fine pyramid shape to obtain high absorbance, and to a method of etching a crystalline silicon wafer using the etching composition.
2. Description of the Related Art
Worldwide, novel renewable energy sources, which are new power sources, have been actively introduced because of fossil energy depletion, rises in the price of oil, climate change, environmental problems and the like. Currently, examples of novel renewable energy sources, which are used as alternatives to source of fossil fuel energy, include solar light, solar heat, wind force, biomass, fuel cells, water power, and the like. Among them, solar energy generation using a solar cell is spreading with the most activity.
A solar cell is a photoelectric cell that converts solar energy into electric energy. Examples of photoelectric cells include a selenium photoelectric cell using the contact between a metal and a semiconductor, a copper sulfite photoelectric cell, a crystalline silicon photoelectric cell using the principle of a P-N junction semiconductor, and the like. Crystalline silicon photoelectric cells are made into a P-N junction semiconductor substrate fabricated by diffusing phosphorus into the surface of a P-type silicon semiconductor, in which boron is added to silicon, to attach an N-type silicon semiconductor to the P-type silicon semiconductor. When an electric field is formed by the P-N junction in such a P-N junction semiconductor substrate and the substrate is irradiated with light such as solar light, electrons (−) and holes (+) are excited, and thus move freely. In this case, when electrons (−) and holes (+) come into the electric field formed by the P-N junction, electrons (−) reach an N-type semiconductor, and holes (+) reach a P-type semiconductor. Here, when electrodes are formed on the surface of the P-type semiconductor and the surface of the N-type semiconductor and electrons flow to an external circuit, electric current is generated. Owing to this principle, solar energy is converted into electric energy. Therefore, in order to maximize the electrical power of a silicon photoelectric cell per unit area, it is required to decrease the reflectance of the silicon photoelectric cell and to maximize its absorbance. For this purpose, the surface of a silicon wafer for a solar cell is textured in a fine pyramid shape and is treated with an antireflective film. The silicon surface textured in the fine pyramid shape decreases the reflectance of incident light across a wide band of wavelengths to increase the intensity of previously-absorbed light, thereby increasing the performance, that is, the efficiency of a solar cell. To date, various methods for texturing the surface of a silicon wafer in a fine pyramid shape have been researched and developed. Specific examples thereof are as follows.
U.S. Pat. No. 4,137,123 discloses a silicon texturing etchant in which 0.5 to 10 wt % of silicon is dissolved in an anisotropic etchant including 0 to 75 wt % of ethylene glycol, 0.05 to 50 wt % of potassium hydroxide and residual deionized water. However, when this silicon texturing etchant is used, there is a problem in that the formation of pyramids on a silicon surface is poor, so that reflectance increases, thereby decreasing the efficiency of a solar cell.
European Patent Publication No. 0477424A1 discloses a texture etching method accompanying an aerating process wherein oxygen is supplied to a texture etching solution in which silicon is dissolved in a solution including ethylene glycol, potassium hydroxide and residual deionized water. However, when this texture etching method is used, there are problems of pyramids being poorly formed on the silicon surface, so that reflectance increases, thereby decreasing the efficiency of the solar cell, and in that an aerator must be additionally provided.
Korean Patent Publication No. 1997-0052617 discloses a texture etching solution including 0.5 to 5 vol % of a potassium hydroxide solution, 3.0 to 20 vol % of isopropyl alcohol and 75 to 96.5 vol % of deionized water. Further, U.S. Pat. No. 6,451,218 discloses a texture etching solution including an alkaline compound, isopropanol, water-soluble alkaline ethylene glycol and residual deionized water. However, these texture etching solutions are problematic in that, since isopropyl alcohol has a low boiling point, isopropyl alcohol must be additionally supplied during a texturing process, so that a large amount of isopropyl alcohol is used, with the result that they are disadvantageous in terms of productivity and economic efficiency, and in that the temperature gradient of the chemical occurs because isopropyl alcohol is supplied during the texturing process, thus deteriorating the texture uniformity of the surface of a silicon wafer.
As described above, the conventional texturing process of a silicon wafer was problematic in that, when only potassium hydroxide is used, a silicon wafer is anisotropically etched, thus increasing the reflectance of the silicon wafer. In order to solve this problem, potassium hydroxide was used in combination with isopropyl alcohol. However, even in this case, there was a problem in that, since the texturing process is conducted at 75 to 80° C., isopropyl alcohol having a boiling point of 80° C. is volatilized, so that isopropyl alcohol must be additionally supplied during the texturing process, with the result that there is a temperature gradient in the chemical, thereby deteriorating the texture uniformity of the surface of the silicon wafer.